Small bathrooms can omit a sprinkler under NFPA 13 in some instances where bathroom walls and ceilings (including behind the shower and tub) are non-combustible or limited combustible with a 15-minute thermal barrier rating (NFPA 13 2002 814.8.1.1, 2007-2016 8.15.8.1.1).

If the bathroom has a window to the exterior, would it need a sprinkler?